Get all the keys from all documents in a Mongo collection

mongo_keys.js

// mapReduce idea credit: http://stackoverflow.com/a/2308036
db.instances.mapReduce(
  function() {
    // map
    for(var key in this) {
      emit(key, null);
    }
  },
  function(key, values) {
    // reduce
    return null;
  },
  {out: {inline: 1}}
).results.map(function(value) {
  // results are `_id`, `value` pairs, where `value` is always `null`
  return value._id;
});